Gas turbine engine components, such as rotor blades and vanes, are used in environments having temperatures approaching or exceeding the allowable temperature limits of the materials used in those components. Cooling fluid is flowed through and over the external surfaces of the components to avoid overheating of the components and its inherent structural degradation. In a typical application, cooling air is flowed through the blade or vane and then ejected through passages extending through to the external surface.
To optimize the effectiveness of the cooling, the cooling passages are angled and shaped to produce a film of cooling fluid over the external surface of the component. An example of one such type of cooling passage is disclosed in U.S. Pat. No. 4,653,983, issued to Vehr and entitled "Cross-Flow Film Cooling Passages". As discussed therein, these passages include a metering section and a diffusing section. The metering section controls the amount of cooling fluid flowing through the passage. The diffusing section reduces the velocity of the ejected fluid to encourage the fluid to form a boundary layer of cooling fluid downstream of the passage. In addition, the diffusing section maximizes the amount of external surface area covered by the film of cooling fluid.
Forming shaped cooling passages in materials such as those used in gas turbine engines presents difficulties. One popular method is to form the passages by electric-discharge machining (EDM). Examples of EDM methods of forming shaped holes are disclosed in U.S. Pat. No. 4,197,443, issued to Sidenstick and entitled "Method and Apparatus for Forming Diffused Cooling Holes in an Airfoil", and U.S. Pat. No. 4,650,949, issued to Field and entitled "Electrode for Electrical Discharge Machining Film Cooling Passages in an Airfoil". EDM provides an easy method to form the complex shape of the diffusing portion while also providing the accuracy required for the metering section.
For many applications, a one step EDM method is sufficient to form the shaped passages. However, for passages having excessive length a one step EDM method may not be economically efficient due to the time intensive nature of the process relative to other available processes, such as laser drilling. In U.S. Pat. No. 4,762,462, issued to Vertz et al and entitled "Airfoil with Diffused Cooling Holes and Method and Apparatus for Making the Same", a two step method of laser drilling and EDM forming a shaped passage is disclosed. This method takes advantage of the speed of the laser drilling step to form the typically longer and simpler shaped metering hole. EDM is then used to form the more complex shape of the diffusing hole.
While less time consuming than the single step EDM methods for lengthy passages, the two step method disclosed in Vertz presents the difficulty of accurately aligning the two portions formed by different processes. Improperly aligning the two portions of the shaped passage leads not only to less effective cooling but may also be a source of crack propagation within the article formed. The latter is especially significant in the gas turbine engine field where the components are highly stressed and under intense heat loads. Cracking reduces the useful life of the gas turbine engine component. Taking into consideration the quantity of cooling passages present in the typical turbine component, a single improperly aligned passage may have costly consequences.
The above art notwithstanding, scientists and engineers under the direction of Applicant's Assignee are working to develop inexpensive and accurate methods and apparatus to form shaped passages in articles.